Abstract:
Growth hormone (GH), thyroxine (T<emph type="8">4) andtri-iodothyronine (T<emph type="8">3) are known to be involved in theregulation of growth and development in a variety of avian species. It hasbeen suggested that an absence of GH and thyroid hormones in ostriches is thecause of their neoteny, a phenomenon in which juvenile characteristics areretained into adulthood. Neoteny is typical of all ratites, the single groupof flightless birds that includes the ostrich, but similar endocrine studieshave not been performed for other members of the group, such as the emu. Totest the neoteny hypothesis further, in the present study we measured theplasma concentrations of T<emph type="8">4, T<emph type="8">3and GH in emus during embryonic development and from hatching to 1 year ofage. Concentrations of T<emph type="8">4 and GH increased during thelast weeks of incubation, whereas concentrations of T<emph type="8">3were highly variable. After hatching, the concentrations of both thyroidhormones were high during the first 3 days of life and then fell to a constantlow level. Plasma concentrations of GH were high at the time of hatching anddecreased gradually over the first 22 weeks of age; thereafter, theconcentrations of GH were highly variable. No correlation was observed betweenhormone concentrations and live weight at any time. These results support thehypothesis that thyroid function is abnormally low in ratites, whereaspatterns of GH secretion are similar to those observed in other birds.Dysfunction of the thyroid axis could explain, in part, the neotenous physicalaspect of adult emus.Extra keywords: development, neoteny, ratite.